This is a single-arm, multicenter, expanded access study of Iodine-131 Anti-B1 Antibody for patients with relapsed or refractory low-grade or transformed low-grade non-hodgkin's B-cell lymphoma (NHL). The primary objective of this study is to make Iodine-131 Anti B-1 Antibody more broadly available to patients. Secondary endpoints of the study will be to obtain additional information on the efficacy and safety of Iodine-131 Anti-B1 Antibody. There will be two dosing phases in this study and then up to two years of follow-up. In the first phase, termed the dosimetric dose, patients will receive an infusion of unlabeled Anti-B1 Antibody (450 mg) over 60 minutes immediately followed by a 30 minute infusion (including a 10-minute flush) of Anti-B1 Antibody (35mg) which has been trace-labeled with 5 mCi of iodine-131. Whole body counts using a gamma camera will be obtained 3 times between Days 0 and 7 following the dosimetric dose. The whole body counts will be used to determine a patient-specific mCi dose of Iodine-131 calculated to deliver the desired total body dose of radiation (either 65 or 75 cGy). The second phase, termed the therapeutic dose, is administered 7-14 days after the dosimetric dose. Patients will again receive an infusion of the Anti-B1 Antibody and then a dose of radiolabeled Antibody in which the dose is dependent on the dosimetric study. Subjects will be treated with an oral iodine solution to protect the thyroid and will continue for 14 days post treatment. Hematology, chemistries and adverse effects will be followed for 13 weeks. Response evaluations will be performed at weeks 13 and 25 and then every 6 months until disease progression, patient deaths, or for two years, whichever occurs first. Thyroid function will also be followed.